208 liev. 0. Fisher — On Faulting, Jointing, and Cleavage. 



level, and X will be lifted 3a with respect to the hidden mass F; 

 so that a thickness of beds equal to 3a will be twice encountered at 

 the vertex of X, which in such a case will form a pointed wedge-like 

 pyramid. 



In natural instances of reversed faults much greater complication 

 usually occurs than with direct faults, because the compression, to 

 which they are due, tends to flex the strata. These are almost 

 always bent at the fault-plane, sometimes on one side of it, sometimes 

 on both, sometimes up, and sometimes down. Sometimes a short 

 cross fault orthogonal to the main fault cuts the ends of the strata, 

 and the beds are bent up against the main fault on one side of 

 this, and down against it on the other. Indeed the parallelism of 

 the strata, which is assumed in the above reasoning, can seldom 

 be invoked in the case of reversed faults. 



PART II. 



The Mechanics of Faulting and Jointing. 



§ I. — Direct Faulting and Jointing. 



There can be no doubt that direct faulting is, in many instances, 

 the consequence of settlement, when the strata contract through, 

 solidification. Let us suppose a certain thickness of sediments to 

 have been deposited upon a bottom, which had already attained its 

 final density. It is evident that the tendency will be, for the 

 sediment, as it solidifies, to contract, both vertically and horizontally. 

 But the layer next to the bottom is held fast by friction, which has 

 there its greatest value, because the pressure is greatest there ; and 

 also by adhesion ; and the surface being horizontal, gravity cannot 

 assist the contractile force to overcome it. In the Pig. (7) let the 

 broken line indicate the height that the sediment would reach if it 

 had not contracted. The tendency will be, for it to settle down, 

 owing to the vertical contraction ; and also to be split up into blocks, 

 owing to the horizontal contraction. But the gaps need not be at any 

 time actually formed, because the internal movements, which we are 

 now considering, will keep pace with the tendency to gape, and 

 prevent any wide gaps from being actually opened. 



The problem then consists in devising the manner of cutting up 

 the mass, consistently with mechanical laws, so that gaps should not 

 be produced during the settlement. The readiest way to conceive 

 how this may happen will be, to suppose them to have been formed, 

 and to consider how they might then be closed. 



Let ABCD be one of 

 the blocks ; and a h C J) 

 the rectangular block of 

 equal volume, into which it 

 must be deformed, in order 

 to fill the two half gaps at 

 either end of it. Then 

 A B Q P is the volume 

 which is available for this 

 purpose. It is to be observed that, since the mass is supposed to 



